There has been known such a kind of electromagnetically operating actuator as described in conventional art documents, for instance, Japanese Patent Application First Publication (KOKAI) No. 8-21220.
Briefly explaining in accordance with FIG. 16, the conventional electromagnetically operating actuator includes an intake valve 2 slidably moveable in a cylinder head 1 of an engine, and an electromagnetically actuating mechanism 3 for actuating the intake valve 2 to be open and closed.
The intake valve 2 includes a valve head 2a opening and closing an open end of an intake port 4, and a valve stem 2b formed integrally with an upper end portion of the valve head 2a.
The electromagnetically actuating mechanism 3 includes a casing 5 fixed onto the cylinder head 1, a disk-like armature 6 fixed to an upper end portion of the valve stem 2b inserted into the casing 5, and a valve-closing electromagnet 7 and a valve-opening electromagnet 8 which are arranged in an inner-upper position and an inner-lower position within the casing 5 and attractively move the armature 6 to close and open the intake valve 2.
Installed between an upper wall of the casing 5 and an upper face of the armature 6 is a valve-opening spring 9 which biases the intake valve 2 in such a direction as to open the intake valve 2. A valve-closing spring 10 is installed between a lower face of the armature 6 and a bottom surface of a spring-seat groove formed on an upper face of the cylinder head 1, which biases the intake valve 2 in such a direction as to close the intake valve 2. Further, the electromagnets 7 and 8 have coils receiving a control current output generated from an electronic control unit 12 via an amplifier 11, respectively.
The electronic control unit 12 is adapted to control an amount of energizing each of the electromagnets 7 and 8 depending on detection signal outputs generated from an engine speed sensor 13 and a temperature detection sensor 14 for the valve-closing electromagnet 7. Reference numeral 15 denotes a power source.
The biasing forces of the two springs 9 and 10 and the attracting forces of the two electromagnets 7 and 8 cooperate together such that the biasing forces are accumulated and retained as a potential energy in the springs 9 and 10 and the electromagnetic forces are alternately repeatedly released from the intake valve 2 and applied thereto. The intake valve 2 is thus actuated to be open and closed.
However, in the conventional electromagnetically operating actuator, at the opening and closing timings of the intake valve 2, the electromagnetically attracting forces of the electromagnets 7 and 8 increase beyond the biasing forces of the springs 9 and 10 which act against the attracting forces, respectively. It is likely that the valve head 2a is caused to strongly conflict with a valve seat 4a at the valve-closing timing and that the armature 6 is caused to conflict with the valve-opening electromagnet 8 at the valve-opening timing.
Referring to FIGS. 17A and 17B, a theory of increase of the attracting forces of the respective electromagnets 7 and 8 is explained. FIG. 17B shows characteristics of the electromagnetically attracting forces and characteristics of the biasing forces of the springs 9 and 10 at the opening and closing timings of the intake valve 2. First, when the intake valve closes, the attracting force of the valve-closing electromagnet 7 causes the armature 6 to move upward. Therefore, the valve-closing spring 10 is expanded as the intake valve 2 slidingly moves upward, while the valve-opening spring 9 is compressed to increase the biasing force to be accumulated therein.
Next, when the intake valve opens, OFF signal (disenergizing state signal) is transmitted to the valve-closing electromagnet 7 while ON signal (energizing state signal) is transmitted to the valve-opening electromagnet 8. The armature 6 is attractively moved downward so that the intake valve 2 slidingly moves downward. Then, the valve-opening spring 9 is expanded, while the valve-closing spring 10 is compressed to increase the biasing force to be accumulated therein.
Accordingly, at the closing and opening timings, the sliding speed of the intake valve 2 decreases due to the increasing biasing forces of the respective valve-opening and valve-closing coil springs 9 and 10. At the time of shifting the intake valve from the closing state to the opening state and vice versa, the attracting force of the electromagnet 7 and 8 in the attracting condition increases abruptly as well as reaction forces of the springs in the compression state and the expansion state. Namely, the electromagnetically attracting forces of the electromagnets 7 and 8 increase in inverse proportion to substantially the square of a distance between the armature 6 and fixed cores 7a and 8a of the electromagnets 7 and 8, respectively. Therefore, the increasing attracting force exceeds the composite biasing force of the springs 9 and 10 conditioned in the compression state and the expansion state, respectively, so that the armature 6 is urged to move quickly upward or downward without adequately reducing the sliding speed. Accordingly, as shown in FIG. 17A, the intake valve 2 abruptly moves up and down at the maximum opening and closing timings. As a result, the valve head 2a abuts on the valve seat 4a at the valve-closing timing, while the armature 6 abuts on the valve-opening electromagnet 8 at the valve-closing timing. In the respective cases, it is likely to cause great strike noise and abrasion on the armature 6 and the valve seat 4a, and the like.
In addition, in the conventional actuator, since the valve head 2a of the intake valve 2 urges the valve seat 4a at a suitable surface pressure, it is required to appropriately balance the attracting force of the valve-closing electromagnet 7 with the biasing force of the valve-opening spring 9. However, there occurs a change in the gap between the armature 6 and the fixed core 7a of the electromagnet 7 due to permanent set of the respective springs 9 and 10 which results from deterioration with age, thermal expansion of the valve stem 2b, abrasion of the valve seat 4a, and the like. This causes a great change in the electromagnetic force. As a result, it will fail to obtain a sufficient retaining force required to maintain the intake valve in the closing state and there will be generated a clearance between the valve head 2a and the valve seat 4a. Then, it is likely that a sealability of the intake valve is reduced and foreign objects such as carbon are accumulated on the seat portion. This tends to deteriorate radiating property of the valve, causing meltdown of the valve.
Further, in the conventional art, when the actuator is mounted onto the cylinder head 1, first the intake valve 2 is inserted into the cylinder head 1 from a lower portion thereof. Subsequently, the valve-opening electromagnet 8 is attached to the upper end portion of the valve stem 2b and then the armature 6 is fixed to the valve stem 2b. That is, since the components of the electromagnetically actuating mechanism 3 must be assembled on the cylinder head 1, the assembly work becomes inconvenient. Particularly, since it is required to accurately place the armature 6 at the upper limit and lower limit positions in order to obtain the appropriate valve-closing retention force as described above, in the assembly work, the working efficiency tends to decrease.